The present invention relates to a loudspeaker apparatus which reproduces audio signals, particularly to a technology suitably applied to a hi-fi reproduction loudspeaker apparatus capable of high-quality sound reproduction.
A variety of constructions have been put into practical use as hi-fi reproduction loudspeaker apparatuses capable of high-quality sound reproduction. For example, there is a loudspeaker apparatus of a three-way structure in which the reproduction band of an audio signal is divided into three bands of a low band, middle band and high band and a loudspeaker unit is individually prepared for each band. This loudspeaker apparatus of a three-way structure becomes capable of accurately reproducing input audio signals from the low band to high band when using, as the above-described loudspeaker for each band, loudspeaker units with favorable reproduction characteristics in respective bands; and typically has a reproduction characteristic more improved than what is called a full-range type loudspeaker unit which outputs audio in all the bands from one loudspeaker unit.
Further, other than the above-described construction in which sound reproduced from a loudspeaker apparatus is made high-quality with a three-way structure or two-way structure, the characteristic of an audio signal itself supplied to a loudspeaker apparatus is corrected on the side of an amplifier and as a result the characteristic of audio output from the loudspeaker apparatus is improved. For example, in some cases, a correction called loudness control is carried out in an audio amplifier that executes processing such as amplification of an audio signal that drives a loudspeaker apparatus. In the loudness control is performed correction processing which makes output levels of the low sound range and the high sound range amplified compared with the middle sound range, and deficiency in the low sound range and the high sound range, which is felt mainly when the sound volume is small, is corrected.
In Published Japanese Patent Application No. 2002-171589, an example of a reproduction structure for carrying out loudness correction is described.
However, regarding reproduced sound on which the loudness control is performed, since signals in a particular frequency band are simply amplified almost uniformly regardless of a level, strictly speaking, reproduction is not performed accurately with respect to input audio signals and therefore development of a loudspeaker apparatus capable of reproducing input audio signals more accurately has been desired. Specifically, regarding reproduced sound on which a loudness control of related art is performed, since sound difficult to be listened to when the sound volume is small is amplified and reproduced, the reproduced sound becomes easy to be listened to and sound quality is improved to some extent in the low sound range and the high sound range in comparison with reproduced sound on which no loudness control is performed; however, since signals in a particular frequency band are uniformly amplified, signal components not requiring amplification may be amplified as well, as a result sound may be reproduced unnaturally.
Here, a problem when sound is reproduced in a loudspeaker apparatus of related art is explained; as an example of a case in which sound is not reproduced accurately with respect to an input audio signal, there is a problem of a small-amplitude signal. Specifically, as shown for example in FIG. 1A, a case is assumed in which an input audio signal S1 whose waveform is a continuation of a waveform of a relatively large amplitude and a waveform of a relatively small amplitude has been input to a loudspeaker. On this occasion, regarding the waveform of an output audio signal S2 from the loudspeaker, a waveform of the relatively large amplitude is almost the same as that of the input signal S1, but a waveform of the relatively small amplitude tends to be smaller in amplitude than that of the input signal S1. This is because, in a loudspeaker unit including a typical diaphragm capable of outputting a relatively large sound, the reproduction characteristic of a signal of a small sound volume having small amplitude is not excellent and so linearity in the input-output characteristic of a signal of a small sound volume is not secured.
Similarly, as shown for example in FIG. 1B, when an input audio signal S3 whose waveform is relatively large in amplitude and an input audio signal S4 whose waveform is relatively small in amplitude overlap temporally, originally both the signals S3 and S4 are combined to be output as an audio signal S5, however, in actuality an output audio signal S6 whose waveform has a level lower than that of the combined signal S5 is output from a loudspeaker. For example, when, as audio reproduced from a loudspeaker, sounds of various musical instruments are simultaneously reproduced as in the case of a symphony, the above output state may arise.
Further, as shown for example in FIG. 1C, when an impulse signal of a specific single frequency in which the amplitude of the signal gradually decreases is input as an input audio signal S7, with respect to the waveform of an audio signal S8 output from a loudspeaker, the following capability deteriorates as the level lowers.
In any example of FIGS. 1A to 1C, regarding output from a loudspeaker, the output level of a signal of a small sound volume having small amplitude becomes smaller than the input signal level, hence the linearity of a small signal is not retained in this state. On analyzing a frequency in the state shown in FIGS. 1A to 1C, a state shown in FIG. 2 is obtained, for example. An example of FIG. 2 is the one in which the sensitivity of a fundamental wave f1 and of harmonic sounds f2 and f3, which are harmonics of the fundamental wave, is analyzed. The fundamental wave f1, whose level is high, is output with its level unchanged, whereas the harmonic sounds f2 and f3, whose levels are smaller than the fundamental wave, have output sensitivities shown by the solid lines below the original levels shown by the broken lines.
FIGS. 3A and 3B are figures showing an output characteristic from the low band to the high band at signal levels of several stages: FIG. 3A is a figure showing an ideal characteristic, and FIG. 3B is a figure showing an output characteristic of a loudspeaker in actuality. As shown in FIG. 3A, in an ideal state, four levels L1, L2, L3 and L4 are positioned at regular intervals and have a flat characteristic from the low band to the high band. On this occasion, an output characteristic of an actual loudspeaker shown in FIG. 3B is as follows: regarding the levels L1, L2 and L3, which are high in output level, almost the same output characteristics as the ideal characteristic are secured; however, regarding the characteristic of the level L4 which is the lowest, it is recognized that the level is lower than an originally required level by a sensitivity α in any frequency band.
FIG. 4 is an input-output characteristic diagram in which such decrease in sensitivity is seen as a characteristic at a specific frequency. As shown in FIG. 4, originally there needs to have a characteristic x shown by the broken line, in which an output level increases linearly as a signal level input to a loudspeaker increases; whereas in actuality there is a characteristic y shown by the curved line, in which above a certain level the level changes almost linearly but output sensitivity with respect to input is considerably unfavorable below a certain level due to a diaphragm less functioning with respect to input.
Specifically, when the maximum level of listening by a typical loudspeaker is assumed to be, for example, 70 to 100 dBspl (sound pressure level), a signal 30 to 60 dB lower than the maximum level will not become (will not be in proportion to) a sound volume accurately 30 to 60 dB lower than the maximum level. If reproduction is assumed to have an output sound volume of an amplifier by 50 dBspl lower than 100 dBspl, a sound volume with 50 dBspl or so can originally be obtained, but in actuality only an output of 40 dBspl that is 10 dBspl lower than that, may be obtained, for example. In other words, it is analyzed and studied by the inventor of the present invention that linearity is inaccurate to be a major cause of the problem in which unsatisfactory sound quality is obtained.
Note that the characteristics explained above are those in the case of a loudspeaker unit with a relatively large diaphragm, capable of outputting a relatively large sound volume; however, on the contrary, in the case of a loudspeaker unit whose diaphragm is small and light-weighted and which is manufactured for outputting a small sound volume such as a loudspeaker unit for a headphone, there is also an apparatus having a structure in which the linearity of an input-output characteristic is relatively favorable regarding a small sound volume, but is not retained favorably regarding a large sound volume.